It is known in Japanese Unexamined Patent Publication No. 10-273494 (Claims, Examples) that phosphites represented by the formula (I) (hereinafter may be referred to as phosphites (I)) are useful as antioxidants for a thermoplastic polymer such as polyolefin.

In the formula (I), R1, R2, R4 and R5 each independently denotes a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, a cycloalkyl group having 5 to 8 carbon atoms, an alkylcycloalkyl group having 6 to 12 carbon atoms, an aralkyl group having 7 to 12 carbon atoms or a phenyl group, and R3 denotes a hydrogen atom or an alkyl group having 1 to 8 carbon atoms. X denotes a single bond, a sulfur atom or a —CHR6 group (R6 denotes a hydrogen atom, an alkyl group having 1 to 8 carbon atoms or a cycloalkyl group having 5 to 8 carbon atoms). A denotes an alkylene group having 1 to 8 carbon atoms or a *—COR7 group (R7 denotes a single bond or an alkylene group having 1 to 8 carbon atoms, and * denotes a bonding hand on the side of oxygen). One of Y and Z denotes a hydroxyl group, an alkoxy group having 1 to 8 carbon atoms or an aralkyloxy group having 7 to 12 carbon atoms, and the other one of Y and Z denotes a hydrogen atom or an alkyl group having 1 to 8 carbon atoms.